A PUTATIVE ZINC-BINDING PROTEIN ON LAMPBRUSH CHROMOSOME LOOPS

We used mAb A33/22, which recognizes a nuclear protein on the loops of amphibian lampbrush chromosomes, to select cDNA clone PwA33 from an expression library of the newt Pleurodeles waltl. A myc-tagged transcript of clone PwA33 was injected into Pleurodeles oocytes. The translation product localized in the germinal vesicle (GV) and was distributed on the lampbrush loops in a pattern identical to that of the endogenous protein. PwA33 encodes a 71 kDa protein with three distinct domains: a region rich in Cys/His residues that may form zinc fingers, a coiled-coil domain with potential for dimerization and a third 'rfp-like' domain that is shared by several other nuclear proteins. The putative zinc fingers and the coiled-coil domain resemble features in known nucleic acid-binding regulatory proteins. These structures, coupled with a distinctive pattern of expression in embryonic tissues, suggest that A33 may function as a regulatory protein during early development. It is unlikely that the large store of A33 in the GV is bound to DNA. Instead, its association with the nascent transcripts on -the lampbrush chromosome loops suggests a role in pre-mRNA synthesis or processing.